/Engine/Plugins/Editor/UVEditor/Source/UVEditor/Public/UVEditorSubsystem.h
The subsystem is meant to hold any UV editor operations that are not UI related (those are handled by the toolkit); however, in our case, most of our operations are wrapped up inside tools and the UV mode. Instead, the subsystem deals with some global UV asset editor things- it manages existing instances (we can't rely on the asset editor subsystem for this because the UV editor is not the primary editor for meshes), and figures out what we can launch the editor on.
[UCLASS](Programming/UnrealArchitecture/Reference/Classes#classdeclaration)() class UUVEditorSubsystem : public [UEditorSubsystem](API\Editor\EditorSubsystem\UEditorSubsystem)
TMap<TObjectPt...
OpenedEditorInstances
TObjectPtr<UTo...
ToolTargetManager
Used to let the subsystem figure out whether targets are valid.